A device of this type is, for example, a loudspeaking telephone set which comprises a different circuit units, such as a speech-controlled switch, a power amplifier and an electronic hybrid circuit.
For their proper operation these circuit units require a certain operating supply voltage. In order to utilize the DC-supply current in the best possible manner for the power amplifier, it is desirable to feed the power amplifier in series with the other circuit units. The operating supply voltages of the circuit units then add up, entailing the risk that a certain maximum, which is still just permissible, is exceeded.
In many automatic telephony systems it is a requirement that the voltage between the connecting terminals of the telephone set in "worst-case" circumstances, at a certain value of the DC-supply current shall not exceed a certain value.
The last-mentioned certain value may be smaller than the sum of the operating supply voltages of the circuit units, so that the said requirement is not satisfied in "worst-case" circumstances.